* Faulty Fuel Level Sender: This is the most probable cause. The sender unit, located in the fuel tank, measures the fuel level and sends a signal to the gauge. If the sender is malfunctioning (often due to corrosion, wear, or a broken float), it might send a consistently high signal, resulting in the gauge staying pegged at full.
* Wiring Issues: A short circuit or break in the wiring between the fuel level sender and the instrument cluster can also cause this problem. Corrosion in the connectors is a frequent culprit.
* Instrument Cluster Problem: While less likely given it affects both cars, a faulty gauge itself in the instrument cluster could be the issue. This is less probable than a sender issue, though.
* Ground Problem: A poor ground connection in the fuel system's electrical circuit can disrupt the signal and lead to inaccurate readings.
Troubleshooting Steps:
1. Check the Fuel Level (Most Important First): This might seem obvious, but *actually* see how much gas is in the tank. If it's nearly empty, the problem is definitely in the fuel gauge system.
2. Inspect the Fuel Level Sender (Requires some mechanical ability): This will likely require dropping the fuel tank (a significant undertaking). Inspect the sender for corrosion, damage to the float, or any obvious wiring problems. You can try gently moving the float arm to see if the gauge needle moves (with the ignition on).
3. Check the Wiring: Carefully examine the wiring harness connecting the fuel sender to the instrument cluster. Look for broken wires, loose connections, corrosion, or any signs of damage. Pay attention to the connectors at both ends.
4. Check Ground Connections: Locate the ground points associated with the fuel gauge system and clean them thoroughly.
5. Test the Fuel Level Sender (Advanced): You could test the sender's resistance using a multimeter. The resistance changes based on the fuel level. This requires a wiring diagram and a bit of electrical knowledge.
6. Instrument Cluster Diagnosis (Last Resort): If all else fails, the issue might lie within the instrument cluster itself. This usually requires professional diagnosis or replacement.
Important Safety Note: Working with fuel tanks involves flammable materials. Always disconnect the battery's negative terminal before starting any work near the fuel tank. Work in a well-ventilated area, and follow all safety precautions. If you're not comfortable doing this yourself, it's best to take your cars to a qualified mechanic.
